The study of the process of differentiation of the olfactory sensory neurons from basal stem cells in the olfactory neuroepithelium of adult mice has provided basic information for the proposed study of neuronal differentiation and maturation in the adult mouse under different experimental circumstances. The proposed work intends to clarify, by morphological and autoradiographic techniques, the importance of the environment on the maturation and senescence process of the olfactory neurons. They are the only neurons, in the mammalian nervous system, to come in direct contact with the external environment. Animals serving as controls, will be kept in sealed cages where filtered, purified air will be circulated. Other animals will be kept in cages where odorous substances at different concentrations will be circulated. Other animals again will be examined after being kept in normal cages under normal laboratory conditions. The rate of stem cells division will be monitored with autoradiographs and the rate of neurons turnover will be determined. The study of a possible circadian rhythm in the mitotic activity of the stem cells will also be studied.